Viruses are adept at immune evasion. For example, they can block specific steps in MHC class I (MHC-I) biosynthesis to thwart cytotoxic T lymphocytes (CTLs). However, host natural killer (NK) cells are better able to kill targets lacking MHC-I, a host fail-safe mechanism described by the missing-self hypothesis which has not been tested during an infection. On the other hand, viruses encode molecules that block NK cell activation by either enhancing the action of MHC-l-specific NK cell inhibitory receptors or blocking the action of NK cell activation receptors, such as NKG2D, which recognizes "stress"-induced ligands. This "arms" race between the pathogen and the host deserves further study because it illuminates how the immune system works in limiting pathogen invasion. In the last funding period, the Yokoyama laboratory collaborated with the Buller laboratory to show that NK cells were important in controlling poxvirus infection. In addition, they initiated studies on cowpox virus (CPXV) that is endemic in rodents. They showed that CPXV uses two open reading frames (ORFs) to down-regulate MHC-I expression. On the other hand, in collaboration with the Carayannopoulos laboratory, they showed that CPXV also encodes a molecule which blocks NKG2Dmediated activation. These findings provide an opportunity to test the missing-self hypothesis in the context of a viral infection. Finally, the poxviruses are distinguished by encoding a large number of other genes that are dispensable for viral growth and replication in vitro;most of these genes probably encode molecules that interact with the host to evade immune reactions. Therefore, the specific aims of this proposal are to: 1) Study viral mechanisms for MHC-I down-regulation. 2) Test the "missing-self" hypothesis by ascertaining the effect of MHC-I down-regulation and/or NKG2D inhibitor on innate and adaptive immune responses during in vivo infections. 3) Explore other viral strategies to evade innate immune responses. Thus, these studies on viral immune evasion will illuminate innate host immune response mechanisms responsible for viral control and will be done in collaboration with Drs. Carayannopoulos and Daved Fremont, among others in the MRCE.